Fluorinated boron hydride anions are weakly coordinating anions and have been used as electrolytes and as catalytic components, particularly to enhance the catalytic activity of metal cations finding use in a variety of reactions.
The substitution of borohydrides, including polyhedral closo-boranes and closo-carboranes with fluorine to various degrees has previously been afforded by employing a number of methods. For example, the direct fluorination of these exemplary boron hydrides with elemental fluorine (F2) have been reported and readily provides highly fluorinated and mixtures of partially fluorinated products. N—F fluorinating agents have also been employed in obtaining mixtures of partially fluorinated products. N—F fluorinating agents suffer from the drawback that they are often accompanied by difficulties associated with impurities formed by the participation and undesirable substitution of the solvent. The fluorination methods listed above, while capable of providing partially fluorinated products, all suffer from the drawback that they typically produce mixtures of products with various isomers and relatively wide ranges of degrees of fluorination.
HF has also been utilized to fluorinate borohydrides. Reactions of B12H122− salts and monocarboranes with HF typically provide an advantage in giving rise to products with narrow ranges for degrees of fluorination. The degree of fluorination is progressive through selective, well-established stereochemistry and may be controlled primarily by varying the reaction temperature such that B12H122− is reported to provide the substitution of only six fluorine atoms at 150-210° C. and monocarboranes are reported to provide the substitution of only 3-4 fluorine atoms in the same temperature range. Additional fluorine atoms can be substituted but under considerably more forcing conditions. These conditions are undesirable for industrial processes because the critical temperature for HF is 188° C. Such high temperature HF is damaging to equipment and dangerous to handle.
Another method for fluorination of boron compounds includes U.S. Pat. No. 3,551,120, which discloses boron compounds of the formula Ma (B12H12−yXy)b where M is a cation having a valence of 1-4, and (B12H12−yXy) is a group which forms a divalent anion in an aqueous solution. The term M represents hydrogen, ammonium, and metal cations, e.g., groups I, II VIII, IIIb and so forth. X represents halogen, (F, Cl, Br, and I), carboxyl, nitro, nitroso, sulfonyl, and so forth. Example 1 shows the formation of Cs2B12H7F5 by effecting fluorination of CsB12H11OH in anhydrous HF. The temperatures utilized by this process are undesirably high in that the temperatures are above the critical temperature for HF.
U.S. Pat. No. 6,180,829 discloses metal compounds of polyhalogenated heteroborane anions of the formula M[RaZBbHcFdXe(OR″)f]k where M is a cation having a valence of from 1-4, e.g., an alkali or alkaline earth metal cation, R typically is a halogen or an alkyl group, Z is C, Si, Ge, Sn, Pb, N, P, As, Sb, and Bi; X is a halide and R″ is a polymer, hydrogen, alkyl and the like. The subscripts represents integers. Example 2 shows the formation of the polyfluorinated monocarborane anion from a monocarborane hydride wherein CsCB11 H12 is reacted with a mixture of HF and 10% F2 in N2. CsCB11F11H was recovered as a white solid. Significant cluster decomposition occurred during the fluorination, and yields were 50-60% at these loadings. U.S. Pat. No. 6,448,447, a continuation-in-part of U.S. Pat. No. 6,180,829 and others, discloses in Example 11 the formation of K2B12F12 (1 g) by the continuous addition of a fluorine/nitrogen gas phase to a suspension of K2B12H12 in HF. This process suffers from the drawback that the distribution of fluorinated products is undersirably broad. In addition, the process utilizes expensive reagents, such as F2.
Knoth et al, Chemistry of Boranes, IX. Fluorination of B10H10−2 and B12H12−2 Inorganic Chemistry, Vol. 2, No. 2, February 1964 disclose the preparation of highly fluorinated dodecaborates, by (a) effecting fluorination with anhydrous HF alone to a composition up to B12F6H62−, and (b) effecting the direct fluorination of a 5 wt. % B12H122− potassium salt by contacting the salt with F2 in the presence of water (under these conditions, the HF concentration is never >10% and thus the Hammett acidity, Ho remains >0 throughout the fluorination). The reaction when conducted in the presence of water was difficult to run to completion as evidenced by the use of a 5-fold excess of fluorine. In the end, a low yield (32%) of a hydroxy substituted fluoroborate, B12F11(OH)2−, was obtained rather than the desired fluorine substituted dodecaborate.
Solntsev, et al, Stereochemical Aspects of the Fluorination of the B12H12−2 Anion, Russian Journal of Coordination of Chemistry, Vol. 23, No. 6, 1997, pp 369-376, disclose that the reaction of supercritical HF with K2B12H12 at 600° C. generates the fully fluorinated anion. Significant decomposition was observed and yields of only 25% were obtained under these conditions.
A stoichiometric oxidative fluorinating agent, antimony pentafluoride (SbF5), has been employed in the fluorination of borohydrides, including o- and m-carboranes. The SbF5 utilized in the fluorination of borohydrides previously known acts as a stoichiometric oxidative fluorinating agent, wherein this role of SbF5 is well documented. The use of superacids, apart from HF alone, in aiding the fluorination of borohydrides is not known in the art.
Thus, what is needed is a process that provides selective and controlled fluorination of borohydrides with facile, high-yielding synthesis of fluorinated boron compounds of narrow and controlled degrees of fluorination under reaction conditions that are industrially practicable. The present invention provides these advantages as well as other related advantages.
The disclosure of the previously identified U.S. Patents is hereby incorporated by reference.